An UE can make UL feedback correctly only when it synchronizes its UL transmission time; therefore, UE has to set up UL sync for receiving DL data from eNB. Otherwise, UE should set up UL sync with the eNB, if it wants to transmit UL data to an eNB.
In RAN51, it was agreed to support the possibility for multiple timing alignment/advance for UL CA in the case of non-collocated site (e.g. Remote Radio Head, RRH) and frequency repeater scenarios. It is supported to transmit signaling and data on several carrier between eNB and UE, according to different UL time, the component carrier that the UE first access successfully is called Pcell (Primary Cell), and other alternative component carriers are called Scell (Secondary Cell). Of course, UE can change the choose of Pcell and Scell correspondingly according to the trigger of eNB. For timing alignment, e.g. UL timing alignment (adapting the UL time), timer advance is used on the UE to offset time delay.
For R10 of 3GPP (Third Generation Partnership Program), UE can only perform random access on Pcell (via PRACH (Physical Random Access Channel)). Because all Scells share one TA value (timing advance value) with Pcell, when UL synchronization for UE's Pcell is missed, UL synchronization for all UE's cells is missed. Therefore, e.g. when DL data arrival but UL synchronization for Pcell is missed, PDCCH (Physical Downlink Control Channel) order signaling is only sent on Pcell by eNB. The PDCCH order signaling comprises a dedicated preamble, known as random access preamble, which is used to indicate the UE to perform contention-free random access. Next, UE will perform random access on Pcell to resume the UL synchronization, after receiving the PDCCH order from eNB. If random access on Pcell is failed, UE will trigger RRC (Radio Resource Control) re-connection procedure.